When Lance Armstrong ran his first marathon in New York City in 2006, he shocked the running world for a few reasons: He clocked a respectable 2:59:36 with little training, and he reportedly ate quite a few chocolate-flavored PowerBar Gels on his run from Staten Island to Tavern on the Green-15 in fact. Stomach churning? Yes. Unheard of? Not necessarily. Many runners are confused about how much fuel they need for a long run, whether in training or racing. Some eat too much, others too little. There are potential perils either way. Having the right long-run nutrition plan can make the difference between finishing strong and not finishing at all.

"What you need are carbohydrates," says Deborah Shulman, Ph.D., a sports nutritionist in Bellvue, Colorado. Carbs are a good source of glucose, a form of sugar that our brain, nerves, and muscles need to function. A small amount of glucose circulates in our blood, but the majority of it is stored in our muscles and liver as glycogen.

The body can store only a limited amount of glycogen. When you deplete your stores, your muscles and brain run out of fuel and you feel physically fatigued and mentally drained. "Hitting the wall" is essentially your brain and muscles running out of carbs. Consuming carbs can help "minimize glycogen depletion and keep blood sugar level," says Shulman. In other words, you'll avoid crashing and burning. On the other hand, if you eat too much midrun, your stomach won't be able to digest all the carbohydrates and you'll probably experience sloshing, bloating, or cramping feelings that signal carb overload.

The 75-Minute Rule

On a run that's about 75 minutes or less, you can rely on your body's glycogen stores and the food you eat prerun to power you through. Run longer, though, and you need carbs.

Jackie Dikos, R.D., a consultant dietitian who heads Nutrition Success in Indianapolis, suggests that runners start "fueling before the onset of fatigue." That means you should start taking in carbs between 30 and 60 minutes into your workout or race, depending on the intensity of your run. Dikos, who ran in this year's Women's Olympic Marathon Trials, starts drinking a carb-rich sports drink about 40 minutes into a marathon. You should then continue fueling in frequent, small doses. The ideal is 100 to 250 calories (or 25 to 60 grams of carbs) per hour, after the first hour of running, says Nancy Clark, M.S., R.D., author of Nancy Clark's Food Guide for Marathoners. That's the equivalent of one to 2 1/2 sports gels or 16 to 40 ounces of sports drink per hour.

That said, a runner's exact calorie needs vary from person to person. As Clark puts it: "A Hummer needs more gas than a Mini Cooper." Smaller runners might only need 100 calories every hour, while larger runners might need around 250 calories. The less fit you are, the faster you burn through stored carbs, meaning you'll need more calories midrun to keep your tank full. Running at a quick pace or high intensity also uses glycogen at a faster rate-a car going 75 miles an hour uses more gas than one going 60.

Many runners rely on sports drinks (Gatorade, Powerade) and gels (PowerBar Gel, GU) for their carbs. "Both are sugar by another name," says Clark. "Sugar is what your body wants." But feel free to eat it in whatever form works for you, whether that's Gummi Bears, dried fruit, or Twizzlers. Clark, a veteran of nine marathons, eats mini Milky Ways on her long runs; Shulman, a runner and triathlete who routinely wins her age group, likes Fig Newtons.

The key to long-run nutrition, says Shulman, is for runners "to experiment with what works for them." Training runs offer the best opportunities to try new carb sources and practice timing your intake. By doing so, you'll learn how much your brain and body need to function at peak levels. And that means no more time lost to pitstops or run-ins with the wall at mile 21.